In heavy industry, military and maritime situations, hooks are provided on a piece of equipment in order to make it more mobile, or to allow for it to be transferred from location to location. In these circumstances, large cranes are utilized, and the chain or cable of the crane is provided with a large loop or ring which is to be engaged with the piece of equipment to be moved. Depending upon the particular use, it may be desirable to have a hook which can be opened either under full load, or without load. One of the common forms of hook available in the industry is a type that, under load, can be opened by use of a long line or chain that actuates a releasing mechanism, and releases the hook when it is under load. The disadvantage of this form is that the hooks are not easy to set or release when not under load. In another form, the action of releasing of the load by placement or by other means automatically releases the hook, and thus terminates the connection between the cable and the device being lifted.
One particular use of this type of equipment is the support of lifeboats aboard ship and on drilling platforms. Lifeboats may comprise enclosed boats that are used on commercial vessels, cruise ships, and off-shore platforms. Twin fall lifeboats are supported by a pair of cables on hoists so that they may be loaded or entered and quickly lowered over the side of a ship or off the side of a platform. Vessels of this type have particular need for a hook locking mechanism which cannot be released under load without substantial inconvenience and the requirement of conscious and deliberate steps to manually release the locking mechanism. This is accomplished by disengaging the coupling to the manual release drive means (e.g., a hand crank for driving the release mechanism) and stowing it in a location separate from the lock release drive mechanism.
Changes in lifeboat launching arrangements have been characterized by slow evolution driven by regulatory change. One change that is particularly relevant was the introduction by the International Maritime Organization (IMO) in 1986 of a regulatory requirement for on-load release hooks. Prior to this time, after lowering a boat into the water, it was necessary manually to unhook the boat from its falls. As boats and their launching gear became larger and heavier, this task had become fraught with danger as crew tried to complete a simultaneous (fore-and-aft) unhooking process. The requirement for on-load release hooks was introduced to overcome these problems, in the expectation that launching would become significantly safer. In practice, on-load release hooks have brought their own problem, with accidents being reported sufficiently frequently for a clear picture to emerge about the types of failure and range of consequences (in terms of seafarer injuries and fatalities) that typically occur. The well-known nature of the problem is illustrated by the publication of two industry surveys. The first was compiled in 1994 by the Oil Companies International Marine Forum (OCINF), based on a questionnaire distributed via the International Chamber of Shipping and selected Flag State Administrations. A total of 92 incidents were identified, 41% of which resulted in injury, with 2 incidents leading to fatalities. OCIMF also noted a lack of confidence amongst mariners leading to reluctance to conduct lifeboat drills. Recommendations were addressed to ship owners, manufacturers and authorities (including the IMO), and it is therefore to be assumed that these various organizations were made aware of the survey findings.
Accident reports make it clear that most accidents to date have occurred during routine drills, maintenance and testing. During these activities, it is usually only members of the ship's crew who are at risk should an accident occur. It also appears that few lifeboat accidents in recent times have occurred during use of the lifeboat in earnest in an emergency abandon ship scenario. The occurrence of serious accidents involving lifeboat on-load release hooks, resulting in injury to or death of seafarers, is an ongoing problem in the shipping industry. Such confidential incident reports highlight both the mechanical problems associated with lifeboat launching arrangements and the resulting lack of confidence amongst seafarers about their safety during lifeboat drills. However, it is evident from the various reports of lifeboat accidents that those involving unexpected or unintended release of the suspension hooks are likely to be the most serious accidents, often leading to fatalities. Preventing or minimizing the occurrence of “hook” accidents would therefore make a major contribution to risk reduction.
In many cases, the failure of on-load hooks is not so much of the hook itself, but more a failure of the release mechanism. To understand the significance of this it is necessary to understand how a typical on-load release hook functions. FIG. 1 (Prior Art) illustrates the working parts of a conventional on-load hook design. Many other manufacturers' designs are believed to operate on equivalent or similar principles. The opening part of the hook may rotate about a swivel pin, which is supported by two side plates of the hook (shown by the long solid line which loops around the top of the swivel pin). The weight of the boat is supported by these side plates, which exert a downward force on the swivel pin. The force is opposed by the tension in the falls, transmitted to the opening part of the hook via the suspension ring. The circular cross section of the suspension ring is seen in the bight of the hook, with an upward force arrow labeled “Hook's load”. The weight of the boat acting downwards at the center of the swivel pin, together with the load in the falls acting upwards at the center of the suspension ring, creates a couple, or an equal and opposite pair of forces acting parallel to each other. This couple tends to rotate the hook in a counter-clockwise direction to open the hook. However, this tendency to open is prevented by the cam.
With further reference to FIG. 1 (Prior Art), the cam comprises a semi-circular shape, wherein an upper part of this cam prevents the hook rotating in a counter-clockwise direction. The cam can rotate about a center of rotation marked “+” in the figure which also shows the hook's tail force pushing on the cam. There is an equal and opposite reaction force from the cam pushing on the tail of the hook. This reaction force acts in a clockwise direction on the hook, balancing the counter-clockwise tendency created by the weight of the boat. The lowest part of the tail of the hook lies above the cam's center of rotation such that if the cam is rotated clockwise around this center, the cam will no longer be in contact with the tail of the hook. Under the influence of a counter-clockwise couple, the hook will open and fall away. Clockwise rotation of the cam is achieved by means of a downwards pull on the cable causing rotation of the cam crank. The cable is connected to the operating lever located adjacent to the coxswain's position in the boat. Since the tail of the hook lies above the cam's center of rotation, the hook's tail force exerts a turning moment on the cam which tends to rotate the cam in a clockwise direction. If allowed to occur, this rotation results in release of the hook. Only the positioning of the cam crank, as dictated by the cable and operating lever, prevents the hook forcing itself open under the action of the couple generated by the boat's weight and tension in the falls.
The above description of the hook design illustrates that many on-load hook designs are inherently “unstable” because the weight of the boat suspended on the hook tends to produce a hook opening effect, which has to be resisted by the operating mechanism for the hook to stay closed. Thus the operating mechanism (lever, cable and cam crank) serves not only to release the boat when required, but also to maintain the hook closed at all other times. Any deficiency in the operating mechanism impacts directly on the ability of the hook to remain closed and support the boat. Thus, many on-load release hooks currently in use are inherently unsafe.
A well-known problem exists with respect to unstable hooks in twin fall lifeboats. This problem was studied in detail by the Maritime and Coastguard Agency (MCA), which concluded that many existing on-load release hooks are inherently unsafe and therefore unfit for use with twin fall lifeboats. The study determined that lifeboat accidents occur for a number of reasons, and that most of the more serious accidents (particularly those involving fatalities), occur because of problems with the on-load release hooks. For example, through the premature or unexpected opening of one or both hooks during a routine test or drill, the lifeboat either becomes suspended vertically or drops completely into the water, frequently resulting in injuries and/or fatalities.
Unsafe situations often arise because many on-load hooks have a tendency to open under the effect of the lifeboat's own weight and need to be closed using an operating mechanism. As a result, there is no defense against: (1) defects/faults in the operating mechanism; (2) errors by the operator; or (3) incorrect resetting of the hook after being released. The MCA concluded that unstable hooks are the primary reason for almost all serious accidents involving lifeboats, and that the solution lies in a radical re-design of the hook types involved. In addition, the MCA recommended that all on-load release hooks be designed and constructed to be stable (i.e., self-closing) when supporting the weight of the lifeboat. Moreover, the MCA suggested that unstable designs of on-load release hooks are identified with the intention that they be withdrawn from service on all ships and urgently replaced with stable designs.